1. Field of the Invention
The invention relates to a plasma processing apparatus where a process is carried out utilizing a plasma onto a substrate such as a semiconductor.
2. Description of the Prior Art
Apparatuses where a process is carried out utilizing a plasma onto a substrate such as a semiconductor wafer and a liquid crystal display (LCD) substrate are well-known as plasma chemical vapor deposition (CVD) apparatuses and plasma etching apparatuses. In these apparatuses, it is necessary to generate high-density plasmas to satisfy a demand for higher process speeds. In addition, it is requested to generate a high-density plasma at a lower pressure from a point of view for enhancing micro-patterning precision. Plasma generation at lower pressures needs improvements such as enhancing ionization efficiency. As apparatuses which can generate a plasma at a low pressure, helicon-wave plasma processing apparatuses, electron cyclotron resonance (ECR) plasma processing apparatuses and inductive-coupled plasma processing apparatuses have been developed.
FIG. 12 shows an example of those prior plasma processing apparatuses, which is disclosed in the Japanese laid-opened No.3-79025. The apparatus shown in FIG. 12 is essentially composed by a vacuum chamber 1 having a dielectric window 11 at its upper wall, a substrate holder 2 for placing a substrate in vacuum chamber 1, a gas introduction means 3 for introducing a plasma generation gas into vacuum chamber 1, a radio frequency (RF) coil 40 provided outside vacuum chamber 1 and close to dielectric window 11, and a RF source 42 for supplying RF power with RF coil 40. Vacuum chamber 1 is an air-tight vessel comprising a pumping system 12. The plasma generation gas is introduced into vacuum chamber 1 by gas introduction means 3. Substrate holder 2 is provided at a lower position inside vacuum chamber 1 and holds a substrate 20 on its upper surface. RF coil 40 is formed like a whirl pool, which axis is vertical to substrate 20, at a plane parallel to substrate 20. The RF power supplied with RF coil 40 from RF source 42 applies a RF field through dielectric window 11. By this RF field, a discharge is ignited with the plasma generation gas introduced into vacuum chamber 1, thus generating a plasma. This plasma is called "inductive-coupled plasma" because RF coil 40 and the plasma are inductively coupled through dielectric window 11. High-density plasmas of order of 10.sup.11 cm.sup.-3, which means 10.sup.11 electrons per one cubic centimeter, at the pressure range of order of 10.sup.-3 Torr can be generated with the apparatus shown FIG. 1. However, this type of apparatus has a problem that high-energy electrons which deteriorate process quality are produced when such a high-density plasma is generated.
Specifically, SiO.sub.2 /Si selective etching using a reactive gas such as C.sub.4 F.sub.8 by a plasma etching method has been studied. This selective etching utilizes a phenomenon where the etching may stop at Si layer because there is no oxygen and carbon polymer film is deposited, contrarily to that the etching does not stop at SiO.sub.2 layer because there is oxygen which produces volatile CO, CO.sub.2 and COF.sub.2 and no carbon polymer film is deposited. Through a study by the inventors, it was turned out that the selectivity of SiO.sub.2 /Si may decrease when the energy supplied with the plasma is increased. The reason of this is supposed that the increased energy produces a lot of high-energy electrons, which may excessively dissociate C.sub.4 F.sub.8 gas. Though the exact mechanism can not be described, it is supposed that the Si layer would be etched because the carbon polymer film is deposited involving fluorine chemical species which are activated by the high-energy electrons, or, the carbon polymer film is deposited under existence of active fluorine or fluoride.
The apparatus shown in FIG. 12 has high probability of production of high-energy electrons which cause the above problem. The reason of this relates to a frequency of the RF energy. The apparatus shown in FIG. 12 uses a frequency of several MHz more than 10 MHz such as 13.56 MHz, and applies an alternating field of this frequency in the plasma generation space. In this case, electrons move through the plasma generation space, changing their directions at every time when the field changes its direction. When some electrons change their moving direction, they follow the field to be sometime accelerated by the field, thus becoming high-energy electrons. To reduce the proportion that electrons follow an alternating field to increase its energy, it is effective to supply RF energy of a frequency of 100 MHz or more. When such a high frequency is used, reversal moving lengths of electrons following the field inversion are shortened. Therefore, the energy increase of electrons is suppressed and the production of high-energy electrons is prevented. However, it is very difficult to excite a RF of 100 MHz or more by a whirl-pool-shaped RF coil shown in FIG. 12.
When a RF of several GHz, i.e., microwave, is used, the RF is coupled by a three dimensional circuit such as a wave guide tube. For example, an ECR plasma processing apparatus uses 2.45 GHz microwave guided by a rectangular wave guide. The production of harmful high-energy electrons, which is demonstrated at several MHz more 10 MHz, is indeed suppressed. On the other hand, it is difficult to generate and maintain a plasma only by RF energy of such a high frequency. In this case, it needs an assistance such as cyclotron resonance by a magnetic field. In fact, ECR plasma processing apparatuses establish the ECR condition applying a high magnetic field of about 1000 gauss its high-density plasma generation However, when a high magnetic field is applied to generate a plasma, there is a problem that a surface process is affected by the magnetic field and loses its uniformity, because electrons are continuously accelerated through mutual reactions of the electric field and the magnetic field. A substrate easily suffers a charging-up damage which pattern corresponds with a magnetic field profile on the substrate surface, resulting from that charged particles are carried along the magnetic lines to the substrate. The charging-up damage is, for example, that breakdown of a insulation film on a substrate surface is caused by a potential difference which corresponds with the magnetic field profile on the substrate surface.
By a study of the inventors, frequency range of 100 MHz to 1000 MHz (1 GHz) can prevent such harmful high-energy electrons from being produced and make it unnecessary to apply such a high magnetic field as in ECR plasma processing apparatuses. However, any practical apparatuses using RF energy of this range have not been developed.